A threshold value of a measurement variable corresponds to the above-mentioned pregiven value for the air/fuel ratio. When the measurement variable is the output voltage of a Nernst probe and the above-mentioned ratio has the value 1, then the threshold value is approximately 450 mV when the probe voltage is directly evaluated. This threshold value can, for example, be so switched that it corresponds to 0 mV or some other suitable value. What is decisive is that, when the probe voltage lies to one side of the threshold voltage, a mixture enrichment is undertaken via the mixture controller; whereas, in the opposite case, the mixture controller undertakes a leaning. If the voltage of a Nernst probe is evaluated directly, the voltages greater than the threshold value correspond to an absolute rich mixture; whereas, voltages below the threshold value correspond to an absolute lean mixture. The position of the threshold voltage is not too critical in the case of a Nernst probe because the Nernst probe exhibits a very large voltage swing, during the transition from rich to lean and vice versa, in a very narrow range of the mixture composition.
When on average, an internal combustion engine is not driven by a stoichiometric mixture but, for example, with a lean mixture, then the threshold value changes correspondingly in order to obtain a switchover between enriching and leaning in another measurement value range. The terms "rich" and "lean" are used herein in a manner so that they apply relative to the threshold value. When utilizing a Nernst probe and the threshold value is set, for example, to only 100 mV, then a measurement value of 200 mV corresponds already to a lean mixture when viewed absolutely; however, further leaning must be undertaken so that the measurement value drops below the threshold value. For this reason, this state is rich referred to the threshold value.
The measurement value must not necessarily be a voltage; instead, the measurement variable can, for example, also be a current such as when a limit-current probe is used in lieu of a Nernst probe in order to measure the mixture composition. The probes can exhibit a high nonlinearity between the content of the oxygen in the mixture and the measurement value as with Nernst probes; or, the relationship can be quite linear as with limit-current probes. The greater the nonlinearity of the relationship, the more intensely do the measures according to the invention become effective.
A mixture controller having a PI characteristic can be utilized with a multi-cylinder internal combustion engine. With this use, the observation can be made that the probe voltage oscillates at a higher frequency than would be expected by the coaction of the PI characteristic and the dead time of the control loop. The dead time is caused, for example, by the transit time of the fuel/air mixture to travel from the intake pipe, be combusted in the engine cylinders and finally arrive at the probe in the exhaust gas output pipe. This effect is characterized as "chemical noise" in U.S. Pat. No. 4,932,383 and is attributed to variations in the composition of the mixture from cylinder to cylinder of the multi-cylinder engine. When various cylinders of an engine are supplied with slightly different mixtures, then the exhaust gas from one of the cylinders can, for example, be slightly lean, whereas, the exhaust gas from another cylinder can still be slightly rich. The measurement value then jumps multiple times above and below the threshold value within a short time interval. Each time, this then immediately triggers a P-jump when a transition from rich to lean is determined. In this way, the output variable can be shifted in an unwanted manner in the lean direction in a very short time. A similar difficulty occurs when a delay time has elapsed after a transition from lean to rich. In all cases, unwanted P-jumps lead to unwanted large control deviations of the mixture composition. SUMMARY OF THE INVENTION
It is an object of the invention to provide a mixture controller for an internal combustion engine which controls the composition of the air/fuel mixture with very slight control deviations.
According to a first embodiment of the invention, a pregiven inhibit time must have elapsed after a first P-jump before a further P-jump is possible in the same direction. According to a second embodiment, a check is made when a delay time has elapsed which had been started when the probe measurement value crossed over the initially-mentioned threshold value in a pregiven direction. This check is as to whether the measurement value still lies on the same side of the threshold voltage as after crossover or not. If this threshold lies on the other side, then no P-jump is triggered.
According to a third embodiment, if the measurement value lies on the same side when the just-mentioned delay time has elapsed, but has been disposed for the greater part of the delay time on the other side, likewise no P-jump is triggered.
In all three embodiments, the technical concept is the same, namely, that a pregiven time span is utilized in order to decide whether a P-jump is to be permitted or not.